2. SOLIDS
Solids
A solid’s particles are packed closely together. The forces between the particles are strong enough that the particles
cannot move freely; they can only vibrate. As a result, a solid has a stable, definite shape and a definite volume. Solids
can only change shape under force, as when broken or cut.
In crystalline solids, particles are packed in a regularly ordered, repeating pattern. There are many different crystal
structures, and the same substance can have more than one structure. For example, iron has a body-centered cubic
structure at temperatures below 912 °C and a face-centered cubic structure between 912 and 1394 °C. Ice has fifteen
known crystal structures, each of which exists at a different temperature and pressure.
A solid can transform into a liquid through melting, and a liquid can transform into a solid through freezing. A solid can
also change directly into a gas through a process called sublimation.
3. LIQUIDS
Liquids
A liquid is a fluid that conforms to the shape of its container but that retains a nearly constant volume independent of
pressure. The volume is definite (does not change) if the temperature and pressure are constant. When a solid is
heated above its melting point, it becomes liquid because the pressure is higher than the triple point of the substance.
Intermolecular (or interatomic or interionic) forces are still important, but the molecules have enough energy to move
around, which makes the structure mobile. This means that a liquid is not definite in shape but rather conforms to the
shape of its container. Its volume is usually greater than that of its corresponding solid (water is a well-known
exception to this rule). The highest temperature at which a particular liquid can exist is called its critical temperature.
A liquid can be converted to a gas through heating at constant pressure to the substance’s boiling point or through
reduction of pressure at constant temperature. This process of a liquid changing to a gas is called evaporation.
4. GASES
Gases
Gas molecules have either very weak bonds or no bonds at all, so they can move freely and quickly. Because of this,
not only will a gas conform to the shape of its container, it will also expand to completely fill the container. Gas
molecules have enough kinetic energy that the effect of intermolecular forces is small (or zero, for an ideal gas), and
they are spaced very far apart from each other; the typical distance between neighboring molecules is much greater
than the size of the molecules themselves.
A gas at a temperature below its critical temperature can also be called a vapor. A vapor can be liquefied through
compression without cooling. It can also exist in equilibrium with a liquid (or solid), in which case the gas pressure
equals the vapor pressure of the liquid (or solid).
A supercritical fluid (SCF) is a gas whose temperature and pressure are greater than the critical temperature and
critical pressure. In this state, the distinction between liquid and gas disappears. A supercritical fluid has the physical
properties of a gas, but its high density lends it the properties of a solvent in some cases. This can be useful in several
applications. For example, supercritical carbon dioxide is used to extract caffeine in the manufacturing of decaffeinated
coffee.
